TCP connection with cellular modem?

I have two machines: A & G. A is connected to the internet via traditional ADSL land-line. G is connected via a GPRS modem (with SIM card).

I can initiate 开发者_StackOverflowSQL connection from G to A and send data, but cannot initiate SQL connection from A to G.

I can also ping from G to A but not otherwise.

is there any limitation on cellular connections ? how to overcome them ?

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Even though, in THEORY, you can ping to G and connect to database services hosted on G, in practice is not always possible for several reasons:

1. On cellular networks, IP addresses rotate (you don't always get the same IP address everytime you connect)
2. On cellular networks, IP addresses are not always reachable/public IP addresses. Sometimes they have a private network and they NAT connections to get to the Internet.

Of course, these two points depend on how your carrier deploys its cellular network, but what I am saying is usually true for a lot of carriers.

If you absolutely MUST connect to a database hosted on the cellular network end I would suggest you try setting up a VPN (OpenVPN is a good product available on many platforms) or using a reverse tunnel (it's simpler, but you will need to setup the reverse tunnel everytime you want to access remote database).